1. Field of the Invention
The present invention relates in general to dispersion-shifted optical fiber of nearly zero chromatic dispersion within a 1.55 .mu.m wavelength band, while achieving reduced non linear effects and low bending loss, and relates in particular to an optical fiber whose dispersion slope is reduced sufficiently.
The present invention is based on a Patent Application No. Hei9-280915, filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Dispersion-shifted optical fiber (referred to as DS-fiber hereinbelow) is an optical fiber whose chromatic dispersion value is almost zero in a 1.55 .mu.m wavelength band where the transmission loss is minimal for quartz group optical fiber. For example, a DS-fiber having a staircase type refractive index distribution (refractive index profile) is well known.
The DS-fiber having such a refractive index profile is characterized by having smaller bending loss compared with other types of DS-fiber, such as step-profile type or triangular-profile type fibers, and somewhat larger mode filed diameter (hereinbelow referred to as MFD); however, relative to the normal single-mode fiber for 1.3 .mu.m band, the MFD is relatively small at about 8 .mu.m or less.
When MFD is small, transmission problems are encountered because not only splice losses are increased but, for applications requiring high power density within the fiber such as optical amplifier applications for example, non linear effects become high and transmission characteristics become seriously degraded.
A quantitative measure of non linear effects is n.sub.2 /Aeff where n.sub.2 is a non linear refractive index for the fiber, and Aeff is the effective cross section area of the fiber. Because n.sub.2 is approximately constant for a given optical material, Aeff must be made large to decrease non linear effects in the fiber.
On the other hand, Aeff and MFD in DS-fibers are related by the following expression: EQU Aeff=k.multidot..pi./4.multidot.(MFD).sup.2
where k is a correction factor.
Here, when the core diameter changed in DS-fibers, there are not less than two radius values for zero chromatic dispersion in the 1.55 wavelength band.
Of these solution values, the smallest value is referred to as the small-diameter solution, and the next smallest value is referred to as the large-diameter solution. Generally, a DS-fiber having staircase type refractive index profile adopts the large-diameter solution.
It has been reported that the correction factor k for a DS-fiber having the normal staircase-type refractive index profile with a large-diameter solution is about 0.944 and remains unchanged regardless of processing parameters used on the fiber.
Therefore, to increase the Aeff, it is necessary to increase MFD.
However, the normal DS-fiber having the staircase type refractive index profile based on the large-diameter solution has a constant MFD value of approximately 8 .mu.m, and therefore, it can not increase the Aeff and enable reduction in non linear effects.
To resolve such problem, the present inventors have proposed a DS-fiber having small-diameter solution in a Japanese Patent Application, First Publication No. Hei8-220362 (Application date Heisei 7, February 10).
In this invention, a small-diameter solution is adopted for DS-filter having a staircase type refractive index profile, thereby increasing the correction factor to about 0.95.about.0.96, and MFD to about 7.8.about.8.6 .mu.m. The result is that Aeff is increased and non linear effects have been reduced.
However, in this invention, although an advantage is gained that Aeff is increased by adopting the small-diameter solution, there remained a difficulty that bending loss and dispersion slope are increased.
Furthermore, for wavelength division multiplexing (WDM) transmission systems, which have been under active development in recent years, even greater reduction in non linear effects is demanded. However, it is difficult for DS-fiber with staircase type refractive index profile to meet such a challenge, because of its limited ability to increase Aeff.
The present inventors have submitted a Japanese Patent Application, Fist Publication, No. Hei10-62640 (application date Heisei 8, August 15), and disclosed a DS-fiber with emphasis on increasing the Aeff.
The DS-fiber disclosed in JPA, First Publication No. Hei10-62640 has a ring-structured refractive index profile, and is comprised by a center core section having a high refractive index, and a ring core section provided separately from the center core section having a low refractive index, cladding provided on the outer periphery of the ring core section, and an intermediate layer disposed between the center core section and the ring core section.
As disclosed in JPA, First Publication, No. Hei8-220362, in DS-fibers having the staircase refractive index profile, it was known that Aeff can be increased by adopting the small-diameter solution. Therefore, in this invention, the small-diameter solution is adopted with a primary objective of increasing Aeff.
The ring-structured DS-fiber (presented in JPA, First Publication No. Hei10-62640) shows almost zero chromatic dispersion in the 1.55 .mu.m band, and its Aeff is higher than that of DS-fiber having the staircase type refractive index profile, thereby resulting in decreased non linear effects as well as low bending loss. Therefore, this type of DS-fiber met two of the important requirements.
However, such a DS-fiber still left a problem that the dispersion slope increases because of the increase in Aeff. High dispersion slope is not desirable in wavelength multiplexing transmission systems because it causes inconsistencies in the transmission of the plural wavelengths.
Accordingly, although increasing in Aeff has been a priority topic, in the past, to reduce non linear effects in DS-fiber for use in wavelength multiplexing system, in recent years, there have been a DS-fiber with achieving sufficient reduction in dispersion slope, rather than with increasing Aeff, to meet the needs of recent system.
Therefore, one of the topics of study in the present invention is to develop a DS-fiber, whose Aeff would be high enough for use in wavelength division multiplexing system to decrease non linear effects, with high priority placed on decreasing its dispersion slope.